This application claims the benefit of the filing date of Taiwan Application Ser. No. 099117507, filed on Jun. 1, 2010 and No. 099146213, filed on Dec. 28, 2010, the contents of which are incorporated herein by reference.
(a) Field of the Invention
The invention relates to a dimmer circuit and a control method.
(b) Description of the Related Art
A conventional TRIAC (Triode for alternating current) dimmer is used to control an incandescent lamp. The TRIAC dimmer outputs an output signal to the incandescent lamp. Further the TRIAC dimmer adjusts phases of the output signal to change the signal output power so as to adjust the luminance of the incandescent lamp. A TRIAC is triggered by a conducting current (latch current Ilatch), and is maintained in a conducting state by a holding current Ihold. Thus a TRIAC should work between the latch current Ilatch and the holding current Ihold for keeping a steady state.
Since a TRIAC works at the the latch current Ilatch and the holding current Ihold, a circuit designed with a TRIAC should couple to a linear load for generating the current Ilatch and Ihold. The incandescent lamp is a linear load so that the TRIAC dimmer may generate the current Ilatch and Ihold with the incandescent lamp. Thus the TRIAC dimmer may function steadily with the incandescent lamp.
Furthermore a LED (light emitting diode) device is not a linear load. The TRIAC dimmer cannot directly drive a LED device to generate the current Ilatch and Ihold. Some external device should be used to generate the Ilatch and Ihold currents.
Generally, a LED device may be driven by the TRIAC dimmer with other external circuits, such as a bleeder. The bleeder may be a linear load for the TRIAC dimmer. However, driving a LED device with the bleeder may cause the following problems:
a. decreasing power usage efficiency and increasing power consumption of circuit;
b. requiring the setting of the Ilatch and Ihold currents to conform to the characteristic of different TRIAC dimmers;
c. using a linear load to provide Ilatch and Ihold to result in temperature increase of the system; and
d. complicate dynamic load circuit design to increase system cost.
Different TRIAC dimmers may be designed with different gate control circuits so as to cause different conduction angles within signals. Further the different conduction angles within signals outputted by the TRIAC dimmer may lead to different output power. Therefore, the design for a LED control circuit has the following problems:
a. different conduction angles between positive and negative conduction angle within an output signal of a TRIAC;
b. different initial conduction angles for a different TRIAC dimmers;
c. difficulty in the linearization of the gate control circuit to cause the difficulty in synchronously dimming the LED control circuit;
d. difficulty in programming a dimming curve for a LED device;
e. abnormal flickering light from a LED device due to a mismatch between characteristic of the TRIAC dimmer and the control circuit of a LED.